One-Two Punch

I have recently come into two books at work that I think anyone who uses CSS should own. Now these are advanced—not for those who are interested in learning CSS at the beginning level. For that, see my future tutorial or go to w3schools; a great resource for web training. For those who have a working knowledge of CSS and want to broaden both their knowledge and their library, these two books are just the thing.
The first book is a reference book: The Ultimate CSS Reference
It will cover your CSS technical knowledge and specs. Not only does this book contain a great compendium of CSS information, but for each selector, or feature, it gives you a chart containing: 1) What version of CSS it belongs to, 2)level of support in the major browsers (great "heads-up" reference for potential Internet Explorer issues), and info on inheritance properties. A very, very handy book and a great tool for web development and browser testing.
It is put out by Sitepoint, another great site for web development knowledge.
So that covers our reference needs. Now, what about expanding your CSS repertoire and furthering your general knowledge? Right hook = "Ultimate CSS Reference", then a left = "CSS Mastery"
Yep, the next book is: CSS Mastery
This book reads more traditionally, as a tutorial and explanatory-type book. It will teach you more advanced techniques and skills in the realm of CSS including greater explanation of the "box property", using CSS with "lists" for navigation, styling forms, and "flickr" style rollover fields on images, among many other things.
Together—one book for reference and one book for advanced CSS techniques—you'll have yourself a winning combo.

"One, two and he's down for the count."